1. Field of the Invention
The present invention relates to an OLED (organic light emitting diode) panel in which an OLED formed over a substrate is sealed between the substrate and a sealing member. The invention also relates to an OLED module obtained by mounting an IC to the OLED panel. In this specification, a light emitting device is used as the generic term for the OLED panel and the OLED module. Also included in the present invention is to electronic appliance using the light emitting device.
2. Description of the Related Art
Being self-luminous, OLEDs eliminate the need for a backlight that is necessary in liquid crystal display devices (LCDs) and thus make it easy to manufacture thinner devices. Also, the self-luminous OLEDs are high in visibility and have no limit in terms of viewing angle. These are the reasons for attention that light emitting devices using the OLEDs are receiving in recent years as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound that provides luminescence (electro luminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use one or both types of light emission.
In this specification, all the layers that are provided between an anode and a cathode is defined as an organic light emitting layer. Specifically, the organic light emitting layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of an OLED is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
One of methods of driving a light emitting device having an OLED is analog driving in which an analog video signal is used.
In analog driving, an analog video signal is inputted to a gate electrode of a TFT that controls a current flowing into the OLED (driving TFT). The amount of drain current of the driving TFT is controlled by the electric potential of the inputted analog video signal. When the drain current flows into the OLED, the OLED emits light at a luminance determined in accordance with the amount of the drain current. A gray scale is thus obtained.
A detailed description will be given with reference to FIG. 19 on how the amount of current supplied to the OLED is controlled by the gate voltage of the driving TFT in the above analog driving.
FIG. 19 is a graph showing a transistor characteristic of the driving TFT. The characteristic is called IDS-VGS characteristic (or IDS-VGS curve). IDS represents the drain current and VGS represents the voltage between the gate electrode and the source region (gate voltage). VTH represents the threshold voltage and V∝ means that VGS is infinite. From the graph, one can tell how much current flows when the gate voltage takes an arbitrary value.
An exclusive relation is formed between gate voltage and drain current in accordance with the IDS-VGS characteristic shown in FIG. 19. In other words, the drain current is determined in accordance with the electric potential of an analog video signal inputted to the gate electrode of the driving TFT. The drain current flows into the OLED, which emits light at a luminance determined in accordance with the amount of the drain current.
When the voltage between the source region and the drain region is given as VDS, the transistor characteristic of the driving TFT which is shown in FIG. 19 is divided into two ranges by values of VGS and VDS. A range where |VGS-VTH|<|VDS| is satisfied is a saturation range, whereas a range where |VGS-VTH|>|VDS| is satisfied is a linear range.
The following equation 1 is satisfied in the saturation range.IDS=β(VGS-VTH)2/2  Equation 1wherein β=μCOW/L, μ represents the mobility, CO represents the gate capacitance per unit area, and W/L represents the ratio of a channel width W to a channel length L of a channel formation region.
As Equation 1 shows, the current value in the saturation range is hardly changed by VDS and is determined solely by VGS. Therefore, it is relatively easy to control the gray scale by the electric potential of an analog signal. Accordingly, the driving TFT is operated mainly in the saturation range in analog driving in general.
In the saturation range, however, a change in gate voltage causes an exponential change in drain current as is apparent in FIG. 19. For that reason, the drain current in analog driving could be changed greatly by the slightest change in gate voltage due to leakage or other causes during a period started with input of an analog video signal and ending with input of the next analog video signal. A great change in drain current is accompanied by a great change in luminance of the OLED. This can therefore lead to a problem of flickering of screen, depending on the frame frequency.
In order to avoid the problem above, it is important to hold the gate voltage securely. Increasing the capacity of the storage capacitor can be one of measures for holding the gate voltage securely. However, when the storage capacitor is increased, the aperture ratio is lowered to reduce the area of a pixel where light emission is actually obtained (area of effective light emission). The term area of effective light emission refers to the area of a region in which light emitted from an OLED is not blocked by objects that do not transmit light, such as a TFT and wiring line formed on the substrate.
In recent years in particular, demands for images of higher definition are increasing and how to solve the problem of lowered aperture ratio which accompanies enhancement of pixel definition is becoming ever important. Accordingly, increasing the area that a storage capacitor occupies in a pixel is not desirable.